Influence of annealing conditions on the structure and luminescent properties of KGd1−xEux(MoO4)2 (0≤ x ≤1)

This paper describes the influence of annealing temperature on the structure and luminescent properties of the KGd1−xEux(MoO4)2 (0≤x≤1). Compounds with the general formula (A’, A’’)n[(W, Mo)O4]m are investigated as luminescent materials for photonic applications, such as phosphor converted LEDs (light-emitting diodes). A KGd0.8Eu0.2(MoO4)2 light-rose crystal was grown by the Czochralski technique. Three polymorphs were present for KGd1−xEux(MoO4)2 in the 923-1223 K range of annealing temperatures at ambient pressure: a triclinic α-phase, an incommensurately modulated monoclinic –phase and an orthorhombic γ-phase with a KY(MoO4)2-type structure. The different behavior of KGd(MoO4)2 and KEu(MoO4)2 is revealed by DSC study. The number and the character of phase transitions for KGd1−xEux(MoO4)2 depend on the elemental composition. The formation of a continuous range of solid solutions with the triclinic α-KEu(MoO4)2–type structure and ordering of K+ and Eu3+/Gd3+ cations is observed only for the α-KGd1−xEux(MoO4)2 (0≤x≤1) prepared at 923 K. The structures of γ-KGd1−xEux(MoO4)2 (x = 0, 0.2) were studied using electron diffraction and refined from powder X-ray diffraction data. The luminescent properties of KGd1−xEux(MoO4)2 prepared at different annealing temperature were studied and related to their different structures. The maxima of the 5D0 → 7F2 integral emission intensities were found under excitation at, respectively, λex = 300 nm and λex = 395 nm for triclinic scheelite-type α-KGd0.6Eu0.4(MoO4)2 and for monoclinic scheelite-type β-KGd0.4Eu0.6(MoO4)2 prepared at 1173 K. The latter shows the brightest red light emission among the KGd1−xEux(MoO4)2 phosphors. The maximum and integral emission intensity of β-KGd0.4Eu0.6(MoO4)2 in the 5D0 → 7F2 transition region is by ~20% higher than that of the commercially used red phosphor of Gd2O2S:Eu3+. Thus, β-KGd0.4Eu0.6(MoO4)2 is very attractive to be applied as a near-UV convertible red-emitting phosphor for LEDs.